Titanium alloys that are light-weight, have a high specific strength, and possess improved corrosion resistance have been utilized in extensive applications such as airplanes and, further, automobile components and civilian goods. Among them, Ti-6Al-4V that is an α+β alloy possessing an improved balance between strength and ductility is a representative example thereof. On the other hand, from the viewpoint of reducing a high cost that is one of factors which are an obstacle to popularization and expansion, alloys having properties that can be alternative to Ti-6Al-4V have been developed using as an additive element Fe that is abundant in resources and is available at a low cost.
α+β titanium alloys can realize an increase in the strength through thermomechanical treatment, but, when the strength increase, generally undergo a lowering in ductility and toughness. However, not only high strength but also high toughness is desired in α+β titanium alloys, because α+β titanium alloys are used, for example, in drives of automobiles and at sites that directly receive impact, such as golf clubs.
Forms of the microscopic structure of the α+β titanium alloy may be classified roughly into an equiaxed structure and an acicular structure. The acicular structure is advantageous in toughness but is poor in strength. In the acicular structure, a fine acicular structure obtained by quenching after solution treatment in a β single-phase area has higher strength and lower toughness than a coarse acicular structure obtained by mild cooling. Further, in the coarse acicular structure, a fatigue fracture is likely to begin at a coarsened α phase, and, thus, the coarse acicular structure is inferior in fatigue strength to the fine acicular structure.
In some cases, in the manufacturing process of Ti-6Al-4V, the cooling rate after the solution treatment in a β single-phase area is increased as a simple means that increases the strength or as a means that increases the productivity on a commercial scale. However, quenching after the solution treatment causes conversion of the microscopic structure to a fine acicular structure, resulting in a significant lowering in toughness of the Ti-6Al-4V alloy.
Ti-6Al-1.7Fe-0.1Si alloys described in Non-Patent Literature 1 and Non-Patent Literature 2 are high-strength and high-rigidity alloys, but on the other hand, the Al addition amount is so large that the toughness is poor.
Patent Literature 1 discloses an alloy consisting of Al: more than or equal to 4.4% and less than 5.5% and Fe: more than or equal to 0.5% and less than 1.4% as an α+β titanium alloy having a fatigue strength that is equal to conventional Ti—Al—Fe-base titanium alloys and that is stable and has little or no variation, and having a higher hot workability than the conventional Ti—Al—Fe-base titanium alloys. The addition amount of Si, however, is less than 0.25% for fatigue strength lowering reasons, and no mention is made of contribution to solid solution strengthening and toughness.
Patent Literature 2 discloses an alloy comprising Al: more than or equal to 4.4% and less than 5.5% and Fe: more than or equal to 1.4% and less than 2.1% as a titanium alloy having a fatigue strength that is equal to conventional Ti—Al—Fe-base titanium alloys, and having a higher hot or cold workability than the conventional Ti—Al—Fe-base titanium alloys. The addition amount of Si, however, is less than 0.25% for fatigue strength lowering reasons, and no mention is made of contribution to solid solution strengthening and toughness.
Patent Literature 3 discloses an alloy consisting of Al: 5.5% to 7.0%, Fe: 0.5% to 4.0%, and O: less than or equal to 0.5% as an α+β titanium alloy that can be manufactured at a low cost on a commercial scale and has mechanical properties more than or equal to Ti-6Al-4V alloys. This alloy, however, disadvantageously has poor toughness due to a large Al addition amount, and suffers from a problem of heterogeneous properties and lowered toughness due to Fe segregation when the Fe content is high.
Patent Literature 4 discloses an titanium alloy consisting of Al: 5.0% to 7.0%, Fe+Cr+Ni: 0.5% to 10.0%, and C+N+O: 0.01% to 0.5% and having a tensile strength of 890 MPa or more and a melting point of 1650° C. or below as cast, as a casting α+β titanium alloy that has a higher strength and a better castability than the Ti-6A-4V. This titanium alloy is an alloy that has good flowability in a melted state and has improved strength after solidification, but is unsatisfactory in strength.
Patent Literature 5 discloses a high-strength α+β alloy that consists of Al: 4.4% to 5.5%, Fe: 1.4% to 2.1%, Mo: 1.5% to 5.5%, and Si: less than 0.1% and has room-temperature strength and fatigue strength more than or equal to Ti-6Al-4V. The titanium alloy described in Patent Literature 5, however, contains a large amount of Mo that is expensive and causes a large price fluctuation, disadvantageously making it difficult to stably manufacture the titanium alloy at a low cost.
Patent Literature 6 discloses a high-strength and high-toughness α+β titanium alloy that has a Mo equivalent of 6.0 to 12.0 and has a controlled microscopic structure. The titanium alloy described in Patent Literature 6 should contain a large amount of Mo that is an expensive alloy element, resulting in a high cost.
Patent Literature 7 discloses a Si-containing near-β titanium alloy. In Patent Literature 7, the near-β titanium alloy is an object alloy, and, like Ti-10V-2Fe-3Al and Ti-5Al-2Sn-2Zr-4Mo-4Cr as exemplified in the specification, V and Mo that are expensive alloy elements are contained in a large amount, thus resulting in a high cost.